With the rapid development of sensor technology, the traditional mechanical flow meter, such as a residential water meter, has been gradually transitioning to a new electronic form. To this end, various sensor technologies have been employed, including optical encoder technology, which can be used to build an absolute angle encoder to enable the production of a direct read or absolute encoder water meter, which does not require an incremental encoder, to measure the amount of liquid flowing through the water meter. Unfortunately this technology suffers from digital errors, resulting from bubbles, light, dirt, leakage and other factors. Compared with optical encoder technology, magnetic encoding technology has higher resolution, no digital error phenomenon, good stability, and can completely eliminate the faults inherent in photovoltaic technology. Magnetic angle encoder technology is a good choice for a direct read water meter, and a good implementation uses magnetoresistive sensors such as tunneling magnetoresistive angular displacement sensors to sense the rotation angle of a magnetic field produced by a cylindrical ring permanent magnet mounted on counting wheels, such that the angle of the field represents the value of the counting wheel, and the angle signal from the sensor is converted to a digital readout using simple electronic components.
Magnetic angle measurement accuracy depends on the performance characteristics of two components of a magnetic angle encoder, and these are the magnetic sensor and the permanent magnet. Compared with Hall sensors, magnetoresistive sensors such as tunneling magnetoresistive sensors have better magnetic field sensitivity, lower power consumption, and smaller size. A tunneling magnetoresistive angular displacement sensor may comprise two mutually orthogonal tunneling magnetoresistive sensors. The tunneling magnetoresistive angular displacement sensor produces two outputs from the magnetic field of the rotating permanent magnet, representing the sine and the cosine of the orientation angle φ of the magnetic field generated by the permanent magnet, and these components can be used to calculate the angle using the following relationships:OUT1=COS(φ)OUT2=SIN(φ)
The inverse tangent function can then be used to calculate the angle φ of the rotating magnetic field from the magnetoresistive angular displacement sensor outputs OUT1 and OUT2:φ=A TAN(OUT2/OUT1).
When the permanent magnet rotates by angle α, the magnetic field produced by the permanent magnet passes through and is detected by the tunneling magnetoresistive sensors located a point defined by r and an angle. When the angle of the magnet α and the angle of the magnetic field φ form a linear relationship in the range of 0˜360°, then the angle φ of the magnetic field detected by the tunneling magnetoresistive sensor represents the mechanical angle α of the permanent magnet. When for example, the 10 positions on a counting wheel are represented by numbers 0-9 on the circumference of the counting wheel and spaced in 10 equal angular increments, defined by dividing the range of α into 10 intervals, then each angular interval represents a specific digit. In this manner by combining a permanent magnet angle encoder with electronics to transform the signals into an appropriate output format, a direct read water meter may be realized.
When tunneling magnetoresistive angular displacement sensor encoder technology is applied to electronic water meter design there will be special performance and design requirements for the permanent magnets, which are difficult to meet with existing magnetic encoder technology due to the following issues:
(1) Existing magnetic angle encoders mostly use Hall sensors to detect the magnetic field generated by the permanent magnet that is perpendicular to the surface of the sensor, and tunneling magnetoresistive sensors detect the magnetic field component parallel to the surface of the sensor, so existing permanent magnet designs are not compatible with the tunneling magnetoresistive sensors.
(2) The existing permanent magnets used for magnetic angle encoders are usually a solid cylindrical design, and this is not compatible with the counting wheels used in typical water meters since the counting wheels are mounted on an axle that needs to pass through the center of each counting wheel and thus through the magnets.